According to DE-C-1520737 (U.S. Pat. No. 3,336,242), PU foams having an open cell structure can be prepared by a one-step process by reacting polyisocyanates with polyoxypropylene-polyoxyethylene-triols having molecular weights of from 3000 to 8000 which contain, in bonded form, from 3 to 5% by weight of ethylene oxide as terminal block and glycerol as initiator molecule.
According to GB-A-1,079,105, very flexible PU foams are obtained from a polyether-polyol or a mixture thereof having a hydroxyl number of from 70 to 170 which contains a trifunctional polyoxyalkylene-polyol, for example propoxylated glycerol, and up to 40% by weight of a polyoxyalkylene glycol, for example propoxylated ethylene glycol, and an organic polyisocyanate, preferably tolylene diisocyante (TDI), in the presence of water and a chlorofluorocarbon, preferably trichlorofluoromethane, as blowing agent.
Flexible PU foams are furthermore described in GB-A-1,064,576, according to which organic diisocyanates, preferably TDI, are reacted with a mixture of from 50 to 90% by weight of a polyoxyalkylene-triol having a hydroxyl number of from 30 to 170 and from 10 to 50% by weight of a polyoxyalkylene-diol having a hydroxyl number of from 40 to 110, the mixture having a content of primary hydroxyl groups of from 10 to 65%, in the presence of water.
Also known is the preparation of diphenylmethane diisocyanate compositions which are liquid at room temperature. According to DE-C-16 18 380 (U.S. Pat. No.3,644,457), these are prepared by reacting one mole of 4,4'- and/or 2,4'-MDI with from 0.1 to 0.3 mol of tri-1,2-oxypropylene glycol and/or poly-1,2-oxypropylene glycol having a molecular weight of up to 700.
According to GB-A-1,369,334, the modification is carried out in two reaction steps, the modifier used being dipropylene glycol or polyoxypropylene glycol having a molecular weight of below 2000.
DE-A-29 13 126 (U.S. Pat. No. 4,229,347) describes MDI compositions in which from 10 to 35% by weight of the isocyanate groups have been reacted with a mixture of at least 3 alkylene glycols, one of these glycols being dipropylene glycol, tripropylene glycol or a higher-molecular-weight polypropylene glycol.
By contrast, the modifier used in DE-A-24 04 166 (GB-A-1,430,455) is a mixture of a polyoxyethylene glycol or polyoxyethylene glycol mixture having a mean molecular weight of less than 650 and at least one alkylene glycol containing at least 3 carbon atoms.
DE-A-23 46 996 (GB-A-1,377,673) relates to MDI compositions in which from 10 to 35% by weight of the isocyanate groups have been reacted with a commercially available polyoxyethylene glycol.
It has also been disclosed that liquid polyisocyanate compositions can be prepared using a mixture of MDI and polyphenyl-polymethylene polyisocyanates (crude MDI) in addition to MDI and glycols and/or polyoxyalkylene glycols.
According to EP-A-10 850, a polyisocyanate composition of this type comprises MDI which has been modified by means of polyoxyalkylene-polyols having a functionality of from 2 to 3 based on polyoxypropylene-polyol and, if desired, polyoxyethylene-polyol having molecular weights of from 750 to 3000, mixed with crude MDI.
According to DE-B-27 37 338 (U.S. Pat. No. 4,055,548), a liquid, crude MDI composition is obtained by combining crude MDI with a polyoxyethylene glycol having a mean molecular weight of from 200 to 600.
According to DE-B-26 24 526 (GB-A-1,550,325), a crude MDI prepared by a special process and containing from 88 to 95% by weight of MDI is reacted with polyoxypropylene glycol having a molecular weight in the range from 134 to 700.
DE-A-25 13 796 (GB-A-1,444,192) and DE-A-25 13 793 (GB-A-1,450,660) relate to crude MDI compositions in which the crude MDI has been modified by means of alkylene glycols or polyoxyalkylene glycols in certain amounts.
Although said alkylene glycols or polyoxyalkylene glycols cause liquefaction of the 4,4'- or 2,4'-MDI isomers, which melt at 42.degree. C. and 28.degree. C. respectively, it is disadvantageous that the polyisocyanate compositions exhibit crystalline deposits after extended storage at temperatures around 10.degree. C.
It is furthermore known to prepare flexible PU foams using crude MDI compositions which have been modified by means of urethane groups as the polyisocyanate component.
According to EP-A-22 617, this preparation process is carried out by reacting a difunctional to trifunctional polyoxypropylene-polyoxyethylene-polyol having a content of polymerized oxyethylene groups of at least 50% by weight with a mixture of MDI isomers, and subsequently diluting the resultant quasi-prepolymer with crude MDI. A particular disadvantage of the PU foams described is their low tensile strength and tear propagation strength.
Polyisocyanate mixtures based on crude MDI which have been modified by means of urethane groups and which have a content of NCO groups of from 12 to 30% by weight are also described in EP-B-0 111 121 (U.S. Pat. No. 4,478,960). The MDI or crude MDI is modified using a polyoxypropylene-polyoxyethylene-polyol having a functionality of from 2 to 4, a hydroxyl number of from 10 to 65 and a content of polymerized ethylene oxide units of from 5 to 30% by weight. These polyisocyanate mixtures which have been modified by means of urethane groups can be used to prepare PU foams having increased elongation at break and improved tensile strength and tear propagation strength. The only disadvantage of these PU foams is that chlorofluorocarbons must be used as blowing agents for their preparation.
Prepolymers containing isocyanate groups, prepared from polyisocyanate mixtures containing 4,4'-MDI and polyoxyalkylene-polyols having a mean functionality of from 2 to 4, a hydroxyl equivalent weight of from 2200 to 3500 and a content of oxyethylene units of from 40 to 68% by weight are, according to EP-A-0 485 953, used for the preparation of flexible PU foams having improved elongation at break.
EP-A-0 555 742 describes a process for the preparation of CFC-free, flexible PU foams using a liquid polyisocyanate mixture containing bonded urethane groups and having a content of NCO groups of from 20 to 30% by weight, which is itself prepared by reacting crude MDI with a polyether-polyol mixture having a hydroxyl number of from 25 to 120 which comprises at least one polyoxypropylene glycol and at least one trifunctional polyoxypropylene-polyoxyethylene-polyol having a content of ethylene oxide units of from 90 to 10% by weight. The flexible PU (molded) foams prepared using these polyisocyanate mixtures which have been modified by means of urethane groups have good mechanical properties, it being possible, depending on the manner in which the polyisocyanate mixtures containing urethane groups have been prepared, for foamable reaction mixtures having better flow properties or flexible PU foams having low tensile strength and elongation, but improved compression set and compressive strength to be obtained.
According to EP-A-0 557 792, flexible, resilient, CFC-free PU (molded) foams of low density can be prepared by reacting starting materials known per se, but using a liquid, MDI-based polyisocyanate mixture which has been modified by means of urethane groups and which has an isocyanate content of from 22 to 30% by weight, which is itself prepared by reacting crude MDI with at least one polyoxypropylene-polyoxyethylene-polyol having a functionality of from 2.5 to 3.5, a hydroxyl number of from 50 to 90 and a content of polymerized ethylene oxide groups of greater than 30 to less than 50% by weight based on the weight of the alkylene oxide groups.
This type of polyisocyanate mixtures containing urethane groups is highly compatible with other formative components for the preparation of flexible PU foams and is readily miscible with polar or nonpolar blowing agents, in particular water and/or carboxylic acids. Said MDI-based polyisocyanate mixtures containing urethane groups and modified by means of polyether-polyols or polyether-polyol mixtures with different structures are suitable for the targeted improvement of at least one specific mechanical property of the flexible PU (molded) foam prepared. However, these publications contain no mention of an increase in resilience or an improvement in the sitting comfort of the flexible PU (molded) foam.
Liquid, pale-colored, MDI-based polyisocyanate mixtures and their use in the preparation of pale-colored, tough and rigid PU foams are described in EP-A-0 550 901.
The prepolymer is prepared using crude MDI containing a proportion of from 75 to 95% by weight of MDI isomers, where the proportion of 2,4'-MDI is from 15 to 40% by weight and the proportion of 2,2'-MDI is from 2 to 10% by weight, and from 5 to 25% by weight of polymeric isocyanates from the MDI series and polyether-polyols having a functionality of from 3 to 8 and a hydroxyl number of from 350 to 1000 mg of KOH/g.
It was hitherto only possible to prepare flexible PU (molded) foams of high sitting comfort using 2,4- and 2,6-tolylene diisocyanate (TDI) isomer mixtures.